Helicopter rotor
Overview
 
A helicopter main rotor or rotor system is a type of fan
Fan (mechanical)
A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...

 that is used to generate both the aerodynamic lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

 force that supports the weight of the helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

, and thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

 which counteracts aerodynamic drag in forward flight. Each main rotor is mounted on a vertical mast over the top of the helicopter, as opposed to a helicopter tail rotor
Tail rotor
The tail rotor, or anti-torque rotor, is a smaller rotor mounted so that it rotates vertically or near-vertically at the end of the tail of a traditional single-rotor helicopter. The tail rotor's position and distance from the center of gravity allow it to develop thrust in the same direction as...

, which is connected through a combination of drive shaft(s) and gearboxes along the tail boom.
Encyclopedia
A helicopter main rotor or rotor system is a type of fan
Fan (mechanical)
A mechanical fan is a machine used to create flow within a fluid, typically a gas such as air.A fan consists of a rotating arrangement of vanes or blades which act on the air. Usually, it is contained within some form of housing or case. This may direct the airflow or increase safety by preventing...

 that is used to generate both the aerodynamic lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

 force that supports the weight of the helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

, and thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....

 which counteracts aerodynamic drag in forward flight. Each main rotor is mounted on a vertical mast over the top of the helicopter, as opposed to a helicopter tail rotor
Tail rotor
The tail rotor, or anti-torque rotor, is a smaller rotor mounted so that it rotates vertically or near-vertically at the end of the tail of a traditional single-rotor helicopter. The tail rotor's position and distance from the center of gravity allow it to develop thrust in the same direction as...

, which is connected through a combination of drive shaft(s) and gearboxes along the tail boom. A helicopter's rotor is generally made up of two or more rotor blades. The blade pitch
Blade pitch
Blade pitch or simply pitch refers to turning the angle of attack of the blades of a propeller or helicopter rotor into or out of the wind to control the production or absorption of power. Wind turbines use this to adjust the rotation speed and the generated power...

 is typically controlled by a swashplate
Swashplate (helicopter)
A swashplate is a device that translates input via the helicopter flight controls into motion of the main rotor blades. Because the main rotor blades are spinning, the swashplate is used to transmit three of the pilot's commands from the non-rotating fuselage to the rotating rotor hub and...

 connected to the helicopter flight controls
Helicopter flight controls
A helicopter pilot manipulates the helicopter flight controls in order to achieve controlled aerodynamic flight. The changes made to the flight controls are transmitted mechanically to the rotor, producing aerodynamic effects on the helicopter's rotor blades which allow the helicopter to be...

.

Helicopter rotor diameters are relatively large, as this gives much better energy and propellant efficiency for the speeds at which helicopters fly.

History and development

Before the development of powered helicopters in the mid 20th century, autogyro
Autogyro
An autogyro , also known as gyroplane, gyrocopter, or rotaplane, is a type of rotorcraft which uses an unpowered rotor in autorotation to develop lift, and an engine-powered propeller, similar to that of a fixed-wing aircraft, to provide thrust...

 pioneer Juan de la Cierva
Juan de la Cierva
Juan de la Cierva y Codorníu, 1st Count of De La Cierva was a Spanish civil engineer, pilot and aeronuatical engineer. His most famous accomplishment was the invention in 1920 of the Autogiro, a single-rotor type of aircraft that came to be called autogyro in the English language...

 researched and developed many of the fundamentals of the rotor. Cierva is credited with successful development of multi-bladed, fully articulated rotor systems. This system, in its various modified forms, is the basis of most multi-bladed helicopter rotor systems.

In the 1930s, Arthur Young
Arthur M. Young
Arthur Middleton Young was an American inventor, helicopter pioneer, cosmologist, philosopher, astrologer and author. Young was the designer of Bell Helicopter's first helicopter, the Model 30, and inventor of the stabilizer bar used on many of Bell's early helicopter designs...

 improved the stability of two-bladed rotor systems with the introduction of a stabilizer bar. This system was used in several Bell and Hiller helicopter
Hiller Aircraft
Hiller Aircraft Company was founded in 1942 as Hiller Industries by Stanley Hiller to develop helicopters.- History :Stanley Hiller, then seventeen, established the first helicopter factory on the West Coast of the United States, located in Berkeley, California, in 1942, under the name "Hiller...

 models. It is also used in many remote control model helicopters
Radio-controlled helicopter
Radio-controlled helicopters are model aircraft which are distinct from RC airplanes because of the differences in construction, aerodynamics, and flight training...

.

Design

A helicopter rotor is powered by the engine, through the transmission, to the rotating mast. The mast is a cylindrical metal shaft which extends upward from—and is driven by—the transmission. At the top of the mast is the attachment point for the rotor blades called the hub. The rotor blades are then attached to the hub. Main rotor systems are classified according to how the main rotor blades are attached and move relative to the main rotor hub. There are three basic classifications: rigid, semirigid, or fully articulated, although some modern rotor systems use an engineered combination of these classifications.

Unlike the small diameter fans used in turbofan
Turbofan
The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...

 jet engines, the main rotor on a helicopter has a quite large diameter, permitting a large volume of air to be accelerated. This permits a lower downwash velocity for a given amount of thrust. As it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree it greatly increases the aircraft's energy efficiency and this reduces the fuel use and permits reasonable range.

Parts and functions

The simple rotor of a Robinson R22
Robinson R22
The Robinson R22 is a two-bladed, single-engine light utility helicopter manufactured by Robinson Helicopter. The two-seat R22 was designed in 1973 by Frank Robinson and has been in production since 1979.-Development:...

 showing (from the top):
  • The following are driven by the link rods from the rotating part of the swashplate
    Swashplate (helicopter)
    A swashplate is a device that translates input via the helicopter flight controls into motion of the main rotor blades. Because the main rotor blades are spinning, the swashplate is used to transmit three of the pilot's commands from the non-rotating fuselage to the rotating rotor hub and...

    .
    • Pitch hinges, allowing the blades to twist about the axis extending from blade root to blade tip.
  • Teeter hinge, allowing one blade to rise vertically while the other falls vertically. This motion occurs whenever translational relative wind is present, or in response to a cyclic control input.
  • Scissor link and counterweight, carries the main shaft rotation down to the upper swashplate
  • Rubber covers protect moving and stationary shafts
  • Swashplates, transmitting cyclic and collective pitch to the blades (the top one rotates)
  • Three non-rotating control rods transmit pitch information to the lower swashplate
  • Main mast leading down to main gearbox

Swash plate

The pitch of main rotor blades can be varied cyclically throughout its rotation in order to control the direction of rotor thrust vector (the part of the rotor disc where the maximum thrust will be developed, front, rear, right side, etc.). Collective pitch is used to vary the magnitude of rotor thrust (increasing or decreasing thrust over the whole rotor disc at the same time). These blade pitch variations are controlled by tilting and/or raising or lowering the swash plate with the flight controls. The vast majority of helicopters maintain a constant rotor speed (RPM) during flight, leaving only the angle of attack of the blades as the sole means of adjusting thrust from the rotor.

The swash plate is two concentric disks or plates, one plate rotates with the mast, connected by idle links, while the other does not rotate. The rotating plate is also connected to the individual blades through pitch links and pitch horns. The non-rotating plate is connected to links which are manipulated by pilot controls, specifically, the collective and cyclic controls.

The swash plate can shift vertically and tilt. Through shifting and tilting, the non-rotating plate controls the rotating plate, which in turn controls the individual blade pitch.

Fully articulated

Juan de la Cierva
Juan de la Cierva
Juan de la Cierva y Codorníu, 1st Count of De La Cierva was a Spanish civil engineer, pilot and aeronuatical engineer. His most famous accomplishment was the invention in 1920 of the Autogiro, a single-rotor type of aircraft that came to be called autogyro in the English language...

 developed the fully articulating rotor for the autogyro
Autogyro
An autogyro , also known as gyroplane, gyrocopter, or rotaplane, is a type of rotorcraft which uses an unpowered rotor in autorotation to develop lift, and an engine-powered propeller, similar to that of a fixed-wing aircraft, to provide thrust...

, and it is the basis of his design that permitted successful helicopter development. In a fully articulated rotor system, each rotor blade is attached to the rotor hub through a series of hinges which allow the blade to move independently of the others. These rotor systems usually have three or more blades. The blades are allowed to flap, feather, and lead or lag independently of each other. The horizontal hinge, called the flapping hinge, allows the blade to move up and down. This movement is called flapping and is designed to compensate for dissymmetry of lift
Dissymmetry of lift
Dissymmetry of lift in rotorcraft aerodynamics refers to an uneven amount of lift on opposite sides of the rotor disc. It is a phenomenon that affects single-rotor helicopters in lateral flight, whether the direction of flight be forwards, sideways or in reverse.The dissymmetry is caused by...

. The flapping hinge may be located at varying distances from the rotor hub, and there may be more than one hinge. The vertical hinge, called the lead-lag or drag hinge, allows the blade to move back and forth. This movement is called lead-lag, dragging, or hunting. Dampers are usually used to prevent excess back and forth movement around the drag hinge. The purpose of the drag hinge and dampers is to compensate for the acceleration and deceleration caused by momentum conservation, and not by Coriolis Effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

. Later models have switched from using traditional bearings to elastomeric bearings.

Rigid

The term "rigid rotor" usually refers to a hingeless rotor system with blades flexibly attached to the hub. Irven Culver of Lockheed developed one of the first rigid rotors, which was tested and developed on a series of helicopters in the 1960s and 1970s. In a rigid rotor system, each blade flaps and drags about flexible sections of the root. A rigid rotor system is mechanically simpler than a fully articulated rotor system. Loads from flapping and lead/lag forces are accommodated through rotor blades flexing, rather than through hinges. By flexing, the blades themselves compensate for the forces which previously required rugged hinges. The result is a rotor system that has less lag in the control response, because the rotor has much less oscillation. The rigid rotor system also negates the danger of mast bumping inherent in semi-rigid rotors.

Semirigid

The semirigid rotor can also be referred to as a teetering or seesaw rotor. This system is normally composed of two blades which meet just under a common flapping, or teetering hinge at the rotor shaft. This allows the blades to flap together in opposite motions like a seesaw
Seesaw
A seesaw is a long, narrow board pivoted in the middle so that, as one end goes up, the other goes down.-Mechanics:Mechanically a seesaw is a lever and fulcrum....

. This underslinging of the blades below the teetering hinge, combined with an adequate dihedral or coning angle on the blades, minimizes variations in the radius of each blade's center of mass from the axis of rotation as the rotor turns, which in turn reduces the stress on the blades from lead and lag forces caused by coriolis effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

. Secondary flapping hinges may also be provided to provide sufficient flexibility to minimize bouncing. Feathering is accomplished by the feathering hinge at the blade root, which allows changes to the pitch angle of the blade.

flybar (Stabilizer bar)

A number of engineers, among them Arthur M. Young
Arthur M. Young
Arthur Middleton Young was an American inventor, helicopter pioneer, cosmologist, philosopher, astrologer and author. Young was the designer of Bell Helicopter's first helicopter, the Model 30, and inventor of the stabilizer bar used on many of Bell's early helicopter designs...

 in the U.S., and Dieter Schlüter in Germany, found that flight stability for helicopters could be achieved with a stabilizer bar or flybar. The flybar has a weight or paddle (or both for added stability on smaller helicopters) at either end which cause the bar to stay relatively stable in the plane of rotation and reduces crosswind thrust on rotors, see flybar forum. Through mechanical linkages, the stable rotation of the bar is mixed with the swashplate movement so that internal (steering) as well as external (wind) forces on the rotor are damped. This eases the workload of the pilot to maintain control of the aircraft. A helicopter with more than two blades does not require a flybar as the extra blades achieve the same result. Stanley Hiller
Stanley Hiller
-Biography:Stanley Hiller was born November 15, 1924 in San Francisco, California to Stanley Hiller, Sr. and Opal Perkins. The family moved to Berkeley, California in the 1930s....

 arrived at a similar method to improve stability by adding short stubby airfoils, or paddles, at each end; However, Hiller's "Rotormatic" system was also used to deliver cyclic control inputs to the main rotor as a sort of control rotor, the paddles provided the added stability by dampening the effects of external forces on the rotor.

In fly-by-wire helicopters or RC models, a microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...

 with gyroscopes and a venturi sensor
Venturi effect
The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The Venturi effect is named after Giovanni Battista Venturi , an Italian physicist.-Background:...

 can replace the stabilizer. This flybar-less design has the advantage of easy reconfiguration and fewer mechanical parts.

Combination

Modern rotor systems may use the combined principles of the rotor systems mentioned above. Some rotor hubs incorporate a flexible hub, which allows for blade bending (flexing) without the need for bearings or hinges. These systems, called "flextures", are usually constructed from composite material. Elastomeric bearings may also be used in place of conventional roller bearings. Elastomeric bearings are bearings constructed from a rubber type material and have limited movement that is perfectly suited for helicopter applications. Flextures and elastomeric bearings require no lubrication and, therefore, require less maintenance. They also absorb vibration, which means less fatigue and longer service life for the helicopter components.

Rotor configurations

Most helicopters have a single, main rotor but require a separate rotor to overcome torque. This is accomplished through a variable pitch, antitorque rotor or tail rotor. This is the design that Igor Sikorsky
Igor Sikorsky
Igor Sikorsky , born Igor Ivanovich Sikorsky was a Russian American pioneer of aviation in both helicopters and fixed-wing aircraft...

 settled on for his VS-300
Vought-Sikorsky 300
|-See also:-References:NotesBibliography* Chiles, James R. The God Machine: From Boomerangs to Black Hawks: The Story of the Helicopter. New York: Bantam, 2008. ISBN 978-0553383522....

 helicopter and it has become the recognized convention for helicopter design, although designs do vary. When viewed from above, the main rotors of helicopter designs from Germany, United Kingdom, The United States and Canada rotate counter-clockwise, all others rotate clockwise. This can make it difficult when discussing aerodynamic effects on the main rotor between different designs, since the effects may manifest on opposite sides of each aircraft.

Single main rotor

With a single main rotor helicopter, the creation of torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

 as the engine turns the rotor creates a torque effect that causes the body of the helicopter to turn in the opposite direction of the rotor. To eliminate this effect, some sort of antitorque control must be used, with a sufficient margin of power available to allow the helicopter to maintain its heading and provide yaw control. The three most common controls used today are the traditional tail rotor, Eurocopter's Fenestron
Fenestron
A Fenestron is a shrouded tail rotor of a helicopter that is essentially a ducted fan. The housing is integral with the tail skin, and, like the conventional tail rotor it replaces, is intended to counteract the torque of the main rotor...

 (also called a fantail), and MD Helicopters
MD Helicopters
MD Helicopters, Inc. is an aerospace company that produces helicopters primarily for commercial use. Coverage here includes the company's tenure as McDonnell Douglas Helicopter Systems, a subsidiary of McDonnell Douglas.-Background:...

' NOTAR
NOTAR
NOTAR is the name of a helicopter anti-torque system which replaces the use of a tail rotor. Developed by McDonnell Douglas Helicopter Systems , the name is an acronym derived from the phrase NO TAil Rotor...

.

Tail rotor

The tail rotor is a smaller rotor mounted so that it rotates vertically or near-vertically at the end of the tail of a traditional single-rotor helicopter. The tail rotor's position and distance from the center of gravity
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...

 allow it to develop thrust in a direction opposite of the main rotor's rotation, to counter the torque effect created by the main rotor. Tail rotors are simpler than main rotors since they require only collective changes in pitch to vary thrust. The pitch of the tail rotor blades is adjustable by the pilot via the anti-torque pedals, which also provide directional control by allowing the pilot to rotate the helicopter around its vertical axis (thereby changing the direction the craft is pointed).


Ducted fan

Fenestron and FANTAIL are trademarks for a ducted fan
Ducted fan
A ducted fan is a propulsion arrangement whereby a fan, which is a type of propeller, is mounted within a cylindrical shroud or duct. The duct reduces losses in thrust from the tip vortices of the fan, and varying the cross-section of the duct allows the designer to advantageously affect the...

 mounted at the end of the tail boom of the helicopter and used in place of a tail rotor. Ducted fans have between eight and 18 blades arranged with irregular spacing, so that the noise is distributed over different frequencies. The housing is integral with the aircraft skin and allows a high rotational speed, therefore a ducted fan can have a smaller size than a conventional tail rotor.

The Fenestron was used for the first time at the end of the 1960s on the second experimental model of Sud Aviation's SA 340, and produced on the later model Aérospatiale
Aérospatiale
Aérospatiale was a French aerospace manufacturer that built both civilian and military aircraft, rockets and satellites. It was originally known as Société Nationale Industrielle Aérospatiale...

 SA 341 Gazelle
Aérospatiale Gazelle
The Aérospatiale Gazelle is a five-seat light helicopter, powered by a single turbine engine. It was designed and manufactured in France by Sud Aviation . It was also manufactured under licence by Westland Aircraft in the United Kingdom , by SOKO in Yugoslavia and ABHCO in Egypt...

. Besides Eurocopter and its predecessors, a ducted fan tail rotor was also used on the canceled military helicopter project, the United States Army
United States Army
The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

's RAH-66 Comanche
RAH-66 Comanche
The Boeing-Sikorsky RAH-66 Comanche was an advanced five-blade armed reconnaissance and attack helicopter designed for the United States Army. The RAH-66 program was canceled in 2004, before mass production began, after nearly $7 billion was spent on the program.During the early 1980s, the U.S...

, as the FANTAIL.

NOTAR

NOTAR, an acronym for NO TAil Rotor, is a helicopter anti-torque system that eliminates the use of the tail rotor on a helicopter. Although the concept took some time to refine, the NOTAR system is simple in theory and works to provide antitorque the same way a wing develops lift using the Coandă effect
Coanda effect
The Coandă effect is the tendency of a fluid jet to be attracted to a nearby surface. The principle was named after Romanian aerodynamics pioneer Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft development....

. A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and driven by the main rotor transmission. This fan forces low pressure air through two slots on the right side of the tailboom, causing the downwash from the main rotor to hug the tailboom, producing lift, and thus a measure of antitorque proportional to the amount of airflow from the rotorwash. This is augmented by a direct jet thruster (which also provides directional yaw control) and vertical stabilizers.

Development of the NOTAR system dates back to 1975, when engineers at Hughes Helicopters
Hughes Helicopters
Hughes Helicopters was a major manufacturer of military and civil helicopters from the 1950s to the 1980s.The company began in 1947, as a unit of Hughes Aircraft, then was part of the Hughes Tool Company after 1955. It became the Hughes Helicopter Division, Summa Corporation in 1972, and was...

 began concept development work. In December 1981, Hughes flew an OH-6A
Hughes H-6
The Hughes OH-6 Cayuse is a single-engine light helicopter with a four-bladed main rotor used for personnel transport, escort and attack missions, and observation...

 fitted with NOTAR for the first time. A more heavily modified prototype demonstrator first flew in March 1986, and successfully completed an advanced flight-test program, validating the system for future application in helicopter design. There are currently three production helicopters that incorporate the NOTAR design, all produced by MD Helicopters. This antitorque design also improves safety by eliminating the possibility of personnel walking into the tail rotor.

Tip jets

Another single main rotor configuration without a tail rotor is the tip jet rotor, where the main rotor is not driven by the mast, but from nozzles on the rotor blade tips; which are either pressurized from a fuselage-mounted gas turbine or have their own turbojet, ramjet
Ramjet
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of airbreathing jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill...

 or rocket
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

 thrusters. Although this method is simple and eliminates torque, the prototypes that have been built are less fuel efficient than conventional helicopters and produced more noise. The Percival P.74
Percival P.74
|-See also:-References:NotesBibliography* Flight via flightglobal archive, 1955.* Winchester, Jim. The World's Worst Aircraft: From Pioneering Failures to Multimillion Dollar Disasters. London: Amber Books Ltd., 2005. ISBN 1-904687-34-2....

 was underpowered and was not able to achieve flight, while the Hiller YH-32 Hornet
YH-32 Hornet
|-See also:-References:* Display information at Museum of Flight in Seattle, Washington* "The First 100 Years of Aviation"-External links:* * *...

 had good lifting capability but performed poorly otherwise. Other aircraft relied on supplemental thrust so that the tipjets could be shut down and the rotor could autorotate after the fashion of an autogyro. The experimental Fairey Jet Gyrodyne
Fairey Jet Gyrodyne
|-See also:-Bibliography:* Charnov, Dr. Bruce H. The Fairey Rotodyne: An Idea Whose Time Has Come – Again? Retrieved: 18 May 2007.* Green, William and Pollinger, Gerald...

 and 40-seat Fairey Rotodyne
Fairey Rotodyne
The Fairey Rotodyne was a 1950s British compound gyroplane designed and built by Fairey Aviation and intended for commercial and military applications...

 passenger prototype were evaluated to have flown very well using this method. Perhaps the most unusual design of this type was the Rotary Rocket Roton ATV
Rotary Rocket
Rotary Rocket, Inc, was a rocketry company headquartered in a facility at Mojave Airport that developed the Roton concept in the late 1990s as a fully reusable single-stage-to-orbit manned spacecraft. Roton was intended to reduce costs of launching payloads into low earth orbit by a factor of...

, which was originally envisioned to take off utilizing a rocket-tipped rotor.An example of a cold jet helicopter is the french Sud-Ouest Djinn, the only tip jet rotorcraft entered into production.

Dual rotors (counterrotating)

Counterrotating rotors are rotorcraft
Rotorcraft
A rotorcraft or rotary wing aircraft is a heavier-than-air flying machine that uses lift generated by wings, called rotor blades, that revolve around a mast. Several rotor blades mounted to a single mast are referred to as a rotor. The International Civil Aviation Organization defines a rotorcraft...

 configurations with a pair or more of large horizontal rotors turning in opposite directions to counteract the effects of torque on the aircraft without relying on an antitorque tail rotor. This allows the power normally required to drive the tail rotor to be applied to the main rotors, increasing the aircraft's lifting capacity. Primarily, there are three common configurations that use the counterrotating effect to benefit the rotorcraft. Tandem rotors are two rotors with one mounted behind the other. Coaxial rotors are two rotors that are mounted one above the other with the same axis. Intermeshing rotors are two rotors that are mounted close to each other at a sufficient angle to allow the rotors to intermesh over the top of the aircraft. Another configuration found on tiltrotors and some earlier helicopters is called transverse rotors where the pair of rotors are mounted at each end of wing-type structures or outriggers.

Tandem

Tandem rotors are two horizontal main rotor assemblies mounted one behind the other. Tandem rotors achieve pitch attitude changes to accelerate and decelerate the helicopter through a process called differential collective pitch. To pitch forward and accelerate, the rear rotor increases collective pitch, raising the tail and the front rotor decreases collective pitch, simultaneously dipping the nose. To pitch upward while decelerating (or moving rearward), the front rotor increases collective pitch to raise the nose and the rear rotor decreases collective pitch to lower the tail. Yaw control is developed through opposing cyclic pitch in each rotor; to pivot right, the front rotor tilts right and the rear rotor tilts left, and to pivot left, the front rotor tilts left and the rear rotor tilts right. All of the rotor power contributes to lift, and it is simpler to handle changes in the center of gravity fore-aft. However, it requires the expense of two large rotors rather than the more common one large main rotor and a much smaller tail rotor. The Boeing CH-47 Chinook is the most common tandem rotor helicopter today.

Coaxial

Coaxial rotors are a pair of rotors mounted one above the other on the same shaft and turning in opposite directions. The advantage of the coaxial rotor is that, in forward flight, the lift provided by the advancing halves of each rotor compensates for the retreating half of the other, eliminating one of the key effects of dissymmetry of lift: retreating blade stall. However, other design considerations plague coaxial rotors. There is an increased mechanical complexity of the rotor system because it requires linkages and swashplates
Swashplate (helicopter)
A swashplate is a device that translates input via the helicopter flight controls into motion of the main rotor blades. Because the main rotor blades are spinning, the swashplate is used to transmit three of the pilot's commands from the non-rotating fuselage to the rotating rotor hub and...

 for two rotor systems. Add that each rotor system needs to be turned in opposite directions means that the mast itself is more complex, and provisions for making pitch changes to the upper rotor system must pass through the lower rotor system.


Intermeshing

Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted on the helicopter with a slight angle to the other so that the blades intermesh without colliding. This configuration is sometimes referred to as a synchropter. Intermeshing rotors have high stability and powerful lifting capability. The arrangement was successfully used in Nazi Germany
Nazi Germany
Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...

 for a small anti-submarine warfare
Anti-submarine warfare
Anti-submarine warfare is a branch of naval warfare that uses surface warships, aircraft, or other submarines to find, track and deter, damage or destroy enemy submarines....

 helicopter, the Flettner Fl 282 Kolibri
Flettner Fl 282
|- References :NotesBibliography* Coates, Steve and Jean-Christophe Carbonel. Helicopters of the Third Reich. Crowborough, UK: Classic Publications Ltd., 2002. ISBN 1-903223-24-5....

. During the Cold War
Cold War
The Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...

, the American company, Kaman Aircraft
Kaman Aircraft
Kaman Aircraft is a U.S. aerospace company, with headquarters in Bloomfield, Connecticut. It was founded in 1945 by Charles Kaman. During the first ten years the company operated exclusively as a designer and manufacturer of several helicopters that set world records and achieved many aviation...

 produced the HH-43 Huskie for the USAF firefighting and rescue missions. The latest Kaman model, the Kaman K-MAX, is a dedicated sky crane design.

Transverse

Transverse rotors are mounted on the end of wings or outriggers, perpendicular to the body of the aircraft. Similar to tandem rotors and intermeshing rotors, the transverse rotor also uses differential collective pitch. But like the intermeshing rotors, the transverse rotors use the concept for changes in the roll attitude of the rotorcraft. This configuration is found on two of the first viable helicopters, the Focke-Wulf Fw 61
Focke-Wulf Fw 61
|-See also:-References:NotesBibliography* Coates, Steve and Jean-Christophe Carbonel. Helicopters of the Third Reich. Crowborough, UK: Classic Publications Ltd., 2002. ISBN 1-903223-24-5....

 and the Focke-Achgelis Fa 223
Focke-Achgelis FA 223 Drache
The Focke-Achgelis Fa 223 Drache was a helicopter developed by Germany during World War II. A single 750 kilowatt Bramo 323 radial engine powered two three-bladed 11.9 metre rotors mounted on twin booms on either side of the 12.2 metre long cylindrical fuselage...

, as well as the world's largest helicopter ever built, the Mil Mi-12. It is also the configuration found on tiltrotor
Tiltrotor
A tiltrotor is an aircraft which uses a pair or more of powered rotors mounted on rotating shafts or nacelles at the end of a fixed wing for lift and propulsion, and combines the vertical lift capability of a helicopter with the speed and range of a conventional fixed-wing aircraft...

s, such the Bell-Boeing V-22 Osprey and the AgustaWestland AW609.


Quadrotor

A quadrotor
Quadrotor
A quadrotor, also called a quadrotor helicopter or quadrocopter, is an aircraft that is lifted and propelled by four rotors. Quadrotors are classified as rotorcraft, as opposed to fixed-wing aircraft, because their lift is derived from four rotors...

 helicopter has four rotors in an "X" configuration designated as front-left, front-right, rear-left, and rear-right. Rotors to the left and right are in a transverse configuration while those in the front and to the rear are in a tandem configuration.

The main attraction of quadrotors is their mechanical simplicity—a quadrotor helicopter using electric motors and fixed-pitch rotors uses only four moving parts.

Blade design

The blades of a helicopter are long, narrow airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....

s with a high aspect ratio
Aspect ratio (wing)
In aerodynamics, the aspect ratio of a wing is essentially the ratio of its length to its breadth . A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings....

, a shape which minimises drag from tip vortices
Wingtip vortices
Wingtip vortices are tubes of circulating air that are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure...

 (see the wings of a glider
Glider (sailplane)
A glider or sailplane is a type of glider aircraft used in the sport of gliding. Some gliders, known as motor gliders are used for gliding and soaring as well, but have engines which can, in some cases, be used for take-off or for extending a flight...

 for comparison). They generally contain a degree of washout
Washout (aviation)
Washout refers to a feature of wing design to deliberately reduce the lift distribution across the span of the wing of an aircraft. The wing is designed so that angle of incidence is higher at the wing roots and decreases across the span, becoming lowest at the wing tip...

 to reduce the lift generated at the tips, where the airflow is fastest and vortex
Wingtip vortices
Wingtip vortices are tubes of circulating air that are left behind a wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and are regions of very low pressure...

 generation would be a significant problem. Rotor blades are made out of various materials, including aluminium, composite structure and steel or titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 with abrasion shields along the leading edge.
Rotorcraft blades are traditionally passive, but research into active blade control trailing edge flaps is performed.

Limitations and hazards

Helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...

s with teetering rotors, for example the two-blade system on the Bell, Robinson
Robinson Helicopter
The Robinson Helicopter Company, based at Zamperini Field in Torrance, California, is the largest manufacturer of civil helicopters in North America.-History:...

  and others, must not be subjected to a low-g condition
Low-G condition
Low-g condition is a phase of aerodynamic flight where the airframe is temporarily unloaded. The pilot—and the airframe—feel temporarily "weightless" because the aircraft is in free-fall or decelerating vertically at the top of a climb. It may also occur during an excessively rapid...

 because such rotor systems do not control the fuselage attitude. This can result in the fuselage assuming an attitude controlled by momentum and tail rotor thrust that causes the tail boom to intersect the main rotor tip-path plane, or result in the blade roots contacting the main rotor drive shaft causing the blades to separate from the hub (mast bumping).

Abrasion in sandy environments

When operating in sandy environments, sand hitting the moving rotor blades erodes their surface. This can damage the rotors; the erosion also presents serious and costly maintenance problems.

The abrasion strips on helicopter rotor blades are made of metal, often titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 or nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

, which are very hard, but less hard than sand. When a helicopter is flown near to the ground in desert environments abrasion occurs from the sand striking the rotor blade. At night, the sand hitting the metal abrasion strip causes a visible corona or halo around the rotor blades. The corona effect is caused by the oxidation of eroded particles resulting in visible corona.

In 2009, war correspondent Michael Yon
Michael Yon
Michael Yon is an American writer and photographer. He served in the Special Forces in the early-1980s, and he became a general freelance writer in the mid-1990s. He focused on military writing after the invasion of Iraq...

referred to this corona effect as "Kopp-Etchells effect", to honor Cpl. Benjamin Kopp, and Cpl. Joseph Etchells, recently fallen American and British soldiers, respectively.

External links

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